1. FIELD OF THE INVENTION
The present invention relates to a semiconductor acceleration sensor and, more particularly, to a semiconductor acceleration sensor which is improved to reduce the influence of acceleration perpendicular to the direction of acceleration to be sensed, thereby enhancing the precision of sensing.
2. DESCRIPTION OF THE RELATED ART
FIG. 3 is a perspective view of a conventional semiconductor acceleration sensor. Referring to this Figure, an acceleration sensor element 1 is partly etched at its reverse side to form a thin-walled diaphragm 2. A plurality of gauge resistors 3a to 3d serving as piezoresistive elements are formed on the upper surface of the diaphragm 2. These gauge resistors have resistance values R.sub.1, R.sub.2, R.sub.4 and R.sub.3.
In operation, the diaphragm 2 is deflected in response to acceleration so that the gauge resistors 3a to 3d are stressed to alter electric signals from which the acceleration is determined. In order to enhance the sensitivity, the acceleration sensor is provided with a weight 4. The acceleration sensor element 1 is cantilevered at one end to a base 6 through a pedestal 5.
In the semiconductor acceleration sensor having the described construction, each of the gauge resistors 3a to 3d increases in resistance when pulled in the longitudinal direction and reduces in resistance when tensioned in the direction perpendicular to the longitudinal direction. These gauge resistors 3a to 3d are connected to form a bridge circuit, an equivalent circuit of which is shown in FIG. 4.
Referring to FIG. 3, it is assumed that the acceleration to be sensed acts in the principal direction (Z-axis direction). When another acceleration is applied to the sensor in a direction (X-axis direction or the direction of another axis) perpendicular to the Z-axis direction, stresses are generated in the respective gauge resistors. More specifically, tensile stresses .sigma..sub.1 and .sigma..sub.2 are generated in the gauge resistors 3a and 3b, respectively, while compressive stresses -.sigma..sub.4 and -.sigma..sub.3 are generated in the gauge resistors 3c and 3d, respectively. The levels of these stresses meet the following conditions: EQU .sigma..sub.1 &gt;.sigma..sub.2 &gt;-.sigma..sub.4 &gt;-.sigma..sub.3 EQU .sigma..sub.1 =.sigma..sub.3 EQU .sigma..sub.2 =.sigma..sub.4
Consequently, the resistance values of the gauge resistors 3a to 3d are respectively changed to R.sub.1 -.DELTA.R.sub.1, R.sub.2 +.DELTA.R.sub.2, R.sub.4 -.DELTA.R.sub.4 and R.sub.3 +.DELTA.R.sub.3. Consequently, the equilibrium state of the bridge circuit is broken to develop a potential difference between the terminals V.sub.out .sym.and V.sub.out .crclbar.. Namely, the following conditions are met. EQU .DELTA.R.sub.1 =.DELTA.R.sub.3 =.DELTA.R.sub.2 =.DELTA.R.sub.4 =.DELTA.R'
Thus, the potential difference, i.e., voltage, generated between the terminals V.sub.out .sym. and V.sub.out .sym. is expressed by the following formula.
(.DELTA.R -R')/R X (input voltage)
In the semiconductor acceleration sensor having the described embodiment, the sensing output includes a component corresponding to the acceleration acting in a direction (X-axis direction) perpendicular to the principal axis (Z-axis), in addition to the component corresponding to the acceleration to be sensed, i.e., the acceleration acting in the direction of the main axis (Z-axis). It is therefore impossible to sense the acceleration acting in the direction of the principal axis precisely.